Apparatus for classifying particulate material



Oct. 31, 1961 J. H. FRANKEN 3,006,470

APPARATUS FOR CLASSIFYING PARTICULATE MATERIAL Filed June 20, 1958 FAN l CYCLONE i GLASSIFIER J00 H. Fran/ en By his af/omeys United States Patent 3,006,470 APPARATUS FOR CLASSIFYING PARTICULATE MATERIAL Jan H. Franken, Heemstede, Netherlands, assignor to Bureau van Tongeren N.V., Heemstede, Netherlands, a company of the Netherlands Filed June 20, 1958, Ser. No. 743,241 6 Claims. (Cl. 209-132) This invention relates to a method and apparatus for classifying particles or granular material into fractions of different particle size.

It is well known to classify particles according to sizes by the use of screens and the like. However, screens have the disadvantage of clogging and wearing due to the abrasive action on the wires of the screen. It is particularly difiicult to effect separation of particles with screens when comparatively small particles are to be separated. For these reasons, various devices have been developed which use aerodynamics to provide the desired separation.

A primary object of the present invention is to provide an improved method and apparatus for classifying particulate material by utilizing a combination of gravitational, inertial, centrifugal and aerodynamic forces.

Another object of this invention is to provide a method and apparatus for classifying materials capable of achieving a sharp division into fractions according to particle size.

These and other objects will become apparent from the description of the drawings in which typical forms of the apparatus are shown and in which like numerals are used to designate like parts throughout.

FIG. 1 is a view in vertical cross section of a preferred embodiment of this invention taken along the line 11 in FIG. 2.

FIG. 2 is a cross section taken along the line 22 in FIG. 1.

FIG. 3 is a diagram illustrating one manner in which this invention may be utilized.

FIG. 4 is a view similar to FIG. 1 except that it shows an alternative form of this invention.

According to the invention, the classifier comprises a chamber, an exhaust duct communicating with the chamber at a side thereof, a plurality of vanes or slats within the exhaust duct and adjacent the chamber, means for moving particulate material in front of said slats, said slats having spaces therebetween such that the fine particles will be entrained in the air passing out of the exhaust duct, and means for establishing an eddy current flow within said chamber in such a way that the eddy current moves in a direction adjacent to the particulate material moving in front of said slats. 7

It is known that fine particles may be separated from coarse particles by virtue of the fact that inertial and gravitational forces are proportional to the mass of the particle which in turn is proportional to the cube of a linear dimension of the particle whereas the drag is directly proportional to the linear dimension. This means that where the linear dimensions are relatively large, as they are in the coarse particles, the inertial and gravitational forces dominate and the particles travel downwardly through'moving air currents. However, in the fine particles the gravitational and inertial forces are small so that the fine particles are picked up by the air going out the exhaust duct and pass through the slats. It should be mentioned, that the slats do not function as a screen, i.e. the space between the slats is much larger than that of the largest linear dimension of the coarse particles. This has the advantage of preventing the clogging which takes place where screening actions are used when the particles have unusual configurations such as fiber forms.

3,006,470 Patented Oct. 31, 1961 The use of gravitational forces to separate coarse particles from entrained fines passing between slats is known, but the use of increased inertial forces which is present in the preferred form of this invention is believed to be novel. The increased inertial forces intensify the effect produced by gravity and tend to provide a sharper separation between coarse and fine particles.

However, there are particles which have a size which is very close to the cut size which are not effectively separated and which tend to increase the absolute fault. Absolute fault is expressed as the percentage of material available in the feed which is improperly classified.

As used herein, the term cut point means the particle that has a reference diameter such that the particle has a 50:50 chance of being found in the fines or in the coarse particles after classification.

This invention provides means whereby those particles having a size close to the cut point are subjected to a plurality of passes in front of the slats and pass upwardly from the bottom of the classifier until they finally settle downwardly with the coarse particles or go outwardly in the exhaust air with the fine particles. This is achieved by the formation of an eddy current within the chamber which has a direction of flow such that the downwardly moving air of the eddy current is at the side of the chamber adjacent to the slats and the upwardly moving air currents are between the center of the chamber and the side opposite the slats.

Preferably these eddy currents are formed by passing air adjacent to the eddy current so that the eddy current is formed by frictional drag.

In the drawings the classifier 10 is generally indicated by the reference character 10 ard the word classifier. There is the chamber 11 (see FIGS. 1 and 4) having two parallel walls 22 and 23 (see FIG. 2) and a partially curved third wall therebetween having a side or portion 24 with a major curve at the top. On the side of the chamber opposite this curved portion 24 of the third wall, there is a substantially vertical side of the chamber, on the left as viewed in FIGS. 1 and 4, containing an exhaust opening 14 and exhaust duct 15 with a series of slats 16. Each slat has its axis extending substantially horizontally and all the axes lying in a substantially vertical plane at the duct 15 and adjacent the opening 14. This vertical side of the third wall of the chamber is structurally related to a primary air inlet duct 18 near the top thereof and a coarse solids outlet 25 at the bottom. The primary air inlet is directed substantially vertically downward and has a feed inlet or conduit 19 bringing the particulate material into the air duct 18 as they pass through an opening 12 into the chamber 11. However, it is important that the particles be in the primary air duct 18 for a length of time sufficient to gain the required increase in velocity. The primary air duct 18 and opening 12 are so positioned that the primary air stream is directed downward in a vertical plane adjacent the opening 14 of chamber 11. Below the feed inlet 19 the opening 14 communicates with an exhaust duct.

The term sla as used herein is intended to include any longitudinal member which presents an inclined surface 17. Thus, in its generic sense the term slat would include rod-shaped or elliptically-shaped members or vanes as well as the preferred flat members shown in the drawing.

In communication with the opening 12 of the chamber 11, there is the primary air inlet duct 18 which is directed substantially vertically downward. The primary air duct 18 and opening 12 are also positioned so that the primary air stream is directed downward in a vertical plane adjacent to the opening 14 of chamber 11 and to slats 16.

Thus the primary air passes vertically downward as v indicated by the arrows 40 inFIG. 1 and turns angularly to become exhaust air as indicated by arrows 41 in the exhaust duct 15. In its preferred form,'the exhaust duct 15 is inclined angularly upward so that the primary air turns at an obtuse angle within the chamber. The efiect of this primary air current 40 is to cause an eddy current 42. to be formed within the classifier by frictional drag, As shown in FIG. 1, the. eddy current 42 has its downwardly moving air between the vertical center line of chamber 11 and the extended axisof the primary air duct 18, while its upwardly moving air begins nearthe bottom of the chamber and travels in a position between the center. of the chamber and the majorv curved side Not the third wall of the chamber.

The material to be classified is supplied into. the primary duct 18 and accelerated downwardly in front of r the slats 16 by the primary air currents. Thus it is vseen that the particulate material to be clas'sifiedis subjected caused by the velocity increase of the particles carried in the air current in the primary air duct 18 as it enters the chamber 11.

Disposedjust below exhaust duct 15, there is a secondary air inlet duct 20; Preferably, there is a common wall 21' .between exhaust duct 15 and'secondary air duct. 20 at the ends of each where they communicate with chamber 11.

The curved'portion 24 of the third wallof the chamher is angularly inclined at the lower portion thereof so that coarser particles which are blown on the wall portion 2 4 will descend downwardly to the bottom of chamber 11; At the bottom of chamber 11, there is a collecting section 25 in which coarse particles are collected. The

collecting section is preferably closed to prevent air from' blowing up into chamber 11 through the collectedcoarse particles. A valve 26 is provided for removal of the coarse particles;

The air passing through the secondary air duct achieves a double eifectas will be apparent by observing its flow. The. path of the air is shown by dotted line 43 of FIG. 1; First of all the secondary air passes through the coarse particles to'provide a scrubbing action and remove any remaining fine particles which are present on the coarse particles. Then the secondary air continues by its inertia over toward the wall portion 24 of chamber .11 and upwardly between the wall portion 24' and the eddy current 42. This action provides additional frictionaldrag on the eddy current 42 and thereby increases the velocity of the eddy current. It is believed that it is novel to provide a clean air current which combines a scrubbing action of the coarse particles with a beneficial effecton the primary classification. In'other words, the secondary air current reinforces or increases the effect of the primary air current in providing the desirable eddy. current 42 which carriesparticles close to the cut point to achieve a sharper cut. Thefine particles which are pickedup'in the secondary air current 43 continue with the air current 43 as it passes around'the'outside of eddy current 42' and then out the exhaust duct'15. The secondary air current not'only carries the entrained fines scrubbed from the coarse particles back to the primary classification, but it also provides means whereby the velocity of air in the exhaust duct' may be controlled. This is important because the velocity'of-air through the exhaust duct will determine-the cut point. Thus, by controlling therelative proportions of primary and secondary a irentering the classifier, theeut'point may be regulated over acomparativelywide range of particle size and still maintain an exceedingly sharp cut.

Inord'er to control the relative proportions of air. entering from primary air duct 18 and secondary air duct 20,

' adamp'er valve 27 may be provided in secondary air duct of the -air stream 'from' thesecondary air duct-"so thatit' passes in a thin layer adjacent to wall21. This causes the secondary air to flow at a high velocity through the particles as they cascade downwardly from wall 21.

It is also considered desirable to utilize comparatively small proportions of secondary air traveling at relatively high velocity. This is important in order to. insure that the secondary air travels in accordancewith the path 43 and to prevent substantial proportions of secondary air from going out the lower port-ion of exhaust duct '15 where it would work against, rather than reinforce, the eddy current. It is also within the scope of this invention to provide additional air ducts which would supply or increase the air current in path 43 to increase the: desired eddy current 42: within the chamber 11. H V

In the operation of the embodimentshown-inFIGS. 1 to 3, the feed particles are allowed to passthroughconduit 19 while primary air passes through primary air duct- 18 and secondary air passes through secondary air duct 20. The particles follow apath substantially asv illus tratedin the drawing,- i.e. they pass downwardly through opening 12 and are then directedvtoward the slats.16 by the passage of air through exhaustduct 15. At the slats, fineparticles are entrained in the exhaust airand pass .out through exhaust duct 15. On the other hand, the intermediate coarse particlesrimpinge against the surfaces-17 ofslats' 16 and are rejected into the coarse particles movingtoward outlet 25. At this point in the/classifier, the slats achieve an excellent classification whereby coarse particles are positively prevented from goingout the exhaust duct 15. However,. some of the fine particles are carried by the coarse particles so that as the mass of particles slide ofr the wal1'21 there are still some fine particles remaining in the coarse fraction. At this point, the secondary air stream sweeps through the coarse particles at a comparatively high velocity and removes substantially all of the fine particles so that only the desired coarse fraction drops downwardly into coarse solids outlet'25.

The secondary air stream also tends to pickup some of the intermediate coarse particles which are in close size relation to the cut point. These intermediate coarse particles tend to settle on Wall portion 24 of chamber'll, and slide by gravity down into. coarse solids outlet 25. The fines entrained in secondary air stream 43 and any smaller coarse particles that may be entrained are carried back into the chamber 11 via airstream43' and are reintroduced into main particle stream.

fines and thenpasses through the feed material to remove additional fines. In this way'the secondary airi'current sweeps through the coarse particles twice in countercurrent relation for removal of fine particles therefrom as well as increasing eddy'current flow.

The fine particles which are not carried out exhaust duct 15 in the first pass are carried'in the eddy current. In general, theeddy current will pick up most of these particles at a point where the main particle streamtimpin'ges on wall 21. be'true regardless of whether there is a secondary air current or not. The particles carried in the eddy current are delivered back to' the primary particle stream by centrifugal force at the primary air iiflet.

The primary air current serves to increase. the downward velocity of the feed material and then exeoutes a turn of say about to carry fi'n'e particles out of the exhaust duct; As. explained above, the "secondary current generally passes into thechambe'r 1'1 and changes direction to move toward the slats so that the secondary air current increases the velocity of the eddy current. By adjusting the control 27 to provide difierent proportions. of controlled prirnaryand secondary air, it is'possible to increase or decrease the'cut point of the classifier. Thus, when a high proportion of secondary.

to primary air provided, moraines are swept out 5 through the exhaust duct 15 to provide a cut point of a comparatively high particle size; while comparatively smaller proportions of secondary air to primary air reduces the cut point. r

'In its process aspect, this invention provides a method of classifying particulate material which comprises, passing the particulate material in a primary air stream in a substantially vertically downward direction in front of a series of slats, said slats having substantially horizontally extending axes all of which lie in a substantially vertical plane, providing a reduced pressure on the side of the slats opposite to the side at which the particulate material is conveyed to carry air and fine particles through said slats, providing a closure whereby substantially all of the primary air goes out through saidslats and causes an eddy current to be formed within the chamber.

In its preferred form, the process aspect of this invention also includes the step of providing a current of sub stantially clean air at a point below the slats such that the clean air passes through the particles falling downwardly into the bottom of the classifier and also directing the current of clean air so that it increases the eddy current formed from frictional drag by the air flow.

In general, the relative proportions of secondary air to primary air will be between and about 40%. The feed rate is not critical and will depend somewhat on the material being classified. There is no theoretical of feed material to air, but the classifier would be inefficient at low rates. For this reason, it is preferred to pass comparatively large proportions of feed material through the classifier.

In FIG. 3, there is shown a diagrammatic view of one setup utilizing the classifier of FIGS. 1 and 2 of this invention. As shown therein, there is a classifier 10, a feed hopper 28 and valve 29 for conveying material to be classified into the duct 19 of the classifier. There is also a cyclone 30 in communication with exhaust duct 15 for the separation of fines which are carried out with exhaust air. The fines are removed from the cyclone and collected in fine collector 31 and the clean exhaust air is dragged through a fan 32 and exhausted into the open air. Section 25 of classifier 10 collects the coarse particles and they are withdrawn therefrom by means of valve 26 and/or valve 33 to coarse particle collector 34. Both the primary air duct 18 and secondary air duct 20 may be open to ambient air pressure and their relative proportions controlled by damper 27 as explained above.

From the above, it is obvious that the only power supply required in this setup is that supplied to the fan 32 and that the system will provide a desired separation of particles into a coarse fraction and a fine fraction. This apparatus achieves an excellent separation when the cut point is selected anywhere from say 50 to 1000 microns.

For example, pulverized limestone was classified utilizing the system illustrated in FIG. 3 with the proportions of and secondary air adjusted to provide a cut point of 150 microns. Twenty-five percent of the feed material was collected in the fine fraction and 75 percent of the feed was collected in the coarse fraction or tailings. Both the fine and coarse fractions were analyzed according to size, and it was found that only 5 percent of the fine material consisted of particles larger than 100 microns and that only about 5 percent of the material in the coarse fraction was less than 100 microns. Thus, the absolute fault was only 5%. In addition, it was found that the improperly classified particles were all substantially close to the cut point in size. Therefore, when it is desired to avoid the presence of coarse particles in the fine fraction, the cut point can be shifted so that a somewhat larger fraction of fines will be in the coarse fraction but substantially no coarse particles will be in the fine fraction. Alternatively, if it is desired to avoid fine contamination in the coarse fraction, the out point may be shifted in the other direction.

Although FIG. 1 shows the primary andsecondary air ducts opening to ambient atmosphere, it is also the scope of this invention to have one or both of these air ducts communicating with a closed system. In fact, as explained above, it is usually preferable to have the air pressure at the secondary air duct higher than that in the primary air duct whereby a comparatively high velocity stream may be provided in the secondary air duct.

In FIG. 4 a modified form of this invention is provided which is particularly useful when the primary air duct communicates with a closed system containing the particles to be classified. As shown therein, the primary air duct carries the particles to be classified throughout its length and vertically downward into the chamber of the classifier. In all other respects, the classifier of FIG. 4 may be similar to the classifier of FIG. 1.

It will be obvious to those skilled in the art that various modifications in form and structure of this apparatus as well as procedural modifications may be made, and it should be understood that the scope of the invention is to be limited only by the scope of the appended claims.

What is claimed is:

1. A classifier comprising walls defining a chamber, a fines exhaust duct connected to said chamber at a side thereof, the lower wall of said exhaust duct being above the bottom of the chamber, an outlet from the chamber below the lower wall of said fines exhaust duct for the coarse fraction, a plurality of slats within said exhaust duct at its opening to the chamber, each of said slats having a substantially horizontal axis and having inclined upper surfaces, a primary air inlet duct positioned at the top of said chamber and at the side of said chamber adjacent to said exhaust duct whereby air flowing through the primary air inlet duct into the chamber and out the exhaust duct causes an eddy current to form within the chamber such that the downwardly flowing portion of the eddy current is on the side of the chamber adjacent the exhaust duct, and means for adding particulate material to be classified to said primary air inlet duct.

2. A classifier for separating particles into fractions having a cut point within the particle size range of from about 50 to 1,000 microns, comprising walls defining a chamber, said chamber having a substantially vertical front side, a rear side wall having its lower portion inclined at an angle, and two substantially vertical side walls, a primary air duct opening into said chamber through the top thereof and adjacent to said vertical front side, means for conveying particulate material into said primary air duct, 2. fluid exhaust duct communicating with said chamber at said substantially vertical side, a secondary air duct communicating with said chamber just below the fluid exhaust duct and above the bottom of the chamber, there being a common wall therebetween at the end where they communicate with the chamber, means for collecting coarse particulate material at the bottom of said chamber, and a plurality of slats disposed within said fluid exhaust duct at a point adjacent to said chamber, said slats being spaced apart at a distance greater than the diameter of the particles being classified and having substantially horizontal axes, said slats also having upper surfaces which are inclined at an angle.

3. A classifier for the division of particulate material in which there is a chamber comprising two parallel walls and a partially curved and partially flat third wall therebetween completing the chamber, there being anopening at the upper part of the third wall, an exhaust duct for the fines in the third wall below the opening and a plurality of slats arranged one above the other across the entrance to the fines exhaust duct, and an inlet duct connected to the opening in the upper part of the third wall adapted to deliver the material to be classified substantially vertically downwardly directly in front of the slats, the third wall having a major curved portion on the opposite side of the inlet duct from the exhaust duct whereby an eddy current is adapted to be set up within the "an. exit. at thetsettem of the chamber :fbr rembval of 'a' warsewfiaction; whereby the upward art e'ftheeiid'y eufrentisadapted' t0 bringany remaining fines aroundto the slhts a' ainiibr exhanst' through the slats after gravity has drcspped a coarse fraction it the exit at the bottom bfth'e' chamber.

4. A; classifier. abcdrding :to claim 3" in which there is zi'secondary air inlet duct dis osed below the fine ext hah'sti duct, being so arranged'that the secondary inlet duct 'is' adapted to direct its air supply with momentum across 'ss'itl' descending. eddy current whereby the eddy flow is strengthened, the coarse particles. are subjected'to a secdnd S01'11bbil1g fi6fi0h and the fines thusw obtained a'i're. carried around to the fines. eXhausLfluct By the: efidy. eurrenth" '5. A classifier as dfihai in laim 3. -ii1-whicfi':c0ntr0 1 means are providedvto adillstrthe reltive'iproportipmof a'i'r nteringone of the inletdnctsp' v I '6; A classifier. as definedlin.clnimAlinwhichlhere is a damper invthe. secondfly air. inlet duct; whereby the 'ain fio'mthe secondary inltduct Hews at adjustable-high velb'cjty through the coarslparficls: as; -they descendbe ReferencjesCitedin the fi lehqf "this. patent UNITED STATE S- PATENTS V 

